Medical device formed of ultrahigh molecular weight polyethylene

ABSTRACT

Medical devices such as catheter balloons, stent covers and vascular grafts formed of ultrahigh molecular weight polyethylene. The devices are formed from polyethylene that has been processed so that it is microporous and has an oriented node and fibril structure. The balloons expand compliantly at low strains and are substantially less compliant at higher strains. The invention also comprises methods for making such balloons, including the steps of compacting a polyethylene powder and deforming it to impart the oriented structure.

BACKGROUND OF THE INVENTION

This invention generally relates to medical devices, and particularly toballoon catheters, stent covers, and vascular grafts.

In percutaneous transluminal coronary angioplasty (PTCA) procedures, aguiding catheter is advanced until the distal tip of the guidingcatheter is seated in the ostium of a desired coronary artery. Aguidewire, positioned within an inner lumen of an dilatation catheter,is first advanced out of the distal end of the guiding catheter into thepatient's coronary artery until the distal end of the guidewire crossesa lesion to be dilated. Then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy, over the previously introduced guidewire,until the balloon of the dilatation catheter is properly positionedacross the lesion. Once properly positioned, the dilatation balloon isinflated with liquid one or more times to a predetermined size atrelatively high pressures (e.g. greater than 8 atmospheres) so that thestenosis is compressed against the arterial wall and the wall expandedto open up the passageway. Generally, the inflated diameter of theballoon is approximately the same diameter as the native diameter of thebody lumen being dilated so as to complete the dilatation but notoverexpand the artery wall. Substantial, uncontrolled expansion of theballoon against the vessel wall can cause trauma to the vessel wall.After the balloon is finally deflated, blood flow resumes through thedilated artery and the dilatation catheter can be removed therefrom.

In such angioplasty procedures, there may be restenosis of the artery,i.e. reformation of the arterial blockage, which necessitates eitheranother angioplasty procedure, or some other method of repairing orstrengthening the dilated area. To reduce the restenosis rate and tostrengthen the dilated area, physicians frequently implant anintravascular prosthesis, generally called a stent, inside the artery atthe site of the lesion. Stents may also be used to repair vessels havingan intimal flap or dissection or to generally strengthen a weakenedsection of a vessel. Stents are usually delivered to a desired locationwithin a coronary artery in a contracted condition on a balloon of acatheter which is similar in many respects to a balloon angioplastycatheter, and expanded to a larger diameter by expansion of the balloon.The balloon is deflated to remove the catheter and the stent left inplace within the artery at the site of the dilated lesion. The frameworkof the stent may still allow migration and proliferation of the smoothmuscle cells, while the stent itself can be thrombogenic. To addressthese problems, stent covers on a surface of the stent have been used.Stent covers have been used in, for example, the treatment ofpseudo-aneurysms and perforated arteries, and to prevent prolapse ofplaque. Similarly, vascular grafts comprising cylinders made from tissueor synthetic materials such as DACRON may be implanted in vessels tostrengthen or repair the vessel, or used in an anastomosis procedure toconnect vessels segments together.

In the design of catheter balloons, balloon characteristics such asstrength, flexibility and compliance must be tailored to provide optimalperformance for a particular application. Angioplasty balloonspreferably have high strength for inflation at relatively high pressure,and high flexibility and softness for improved ability to track thetortuous anatomy and cross lesions. The balloon compliance is chosen sothat the balloon will have a desired amount of expansion duringinflation. Compliant balloons, for example balloons made from materialssuch as polyethylene, exhibit substantial stretching upon theapplication of tensile force. Noncompliant balloons, for exampleballoons made from materials such as PET, exhibit relatively littlestretching during inflation, and therefore provide controlled radialgrowth in response to an increase in inflation pressure within theworking pressure range.

For many applications, intravascular catheter balloons should besubstantially noncompliant once expanded to a working diameter. Further,catheter balloons should also be formed from relatively strong materialsin order to withstand the pressures necessary for various procedureswithout failing. Typically, such characteristics require the use of amaterial that does not stretch, which consequently necessitates that theballoon material be folded around the catheter shaft prior to inflation.However, it can be desirable to employ balloons that are not foldedprior to inflation, but which are instead expanded to the workingdiameter from a generally cylindrical shape having a nominal diameterthat conforms to the catheter shaft. Such designs may be used forformed-in-place angioplasty balloons and stent delivery balloons. Priorart formed-in-place balloons have suffered from problems such asinsufficient strength, poor control over expansion, and significantlycomplicated processing during catheter manufacturing.

It would be a significant advance to provide a catheter balloon, andother expandable members such as stent covers, and vascular grafts, withimproved processing and expansion characteristics.

SUMMARY OF THE INVENTION

This invention is directed to medical devices having at least acomponent formed of ultrahigh molecular weight polyethylene (hereinafter “UHMW polyethylene”). In a presently preferred embodiment, theUHMW polyethylene is microporous with a node and fibril microstructurecomprising nodes interconnected by fibrils. One embodiment of theinvention comprises a balloon for an intraluminal catheter, formed atleast in part of the UHMW polyethylene. In another embodiment of theinvention, a stent delivery system comprising a balloon catheter and astent mounted on the balloon has a component, such as the catheterballoon or a stent cover, which is formed at least in part of the UHMWpolyethylene. Another embodiment of the invention comprises a vasculargraft formed at least in part of the UHMW polyethylene. Althoughdiscussed below primarily in terms of a balloon catheter having aballoon formed of UHMW polyethylene, the invention should be understoodto include other medical devices such as stent covers and vasculargrafts formed of UHMW polyethylene.

The UHMW polyethylene has a molecular weight which is higher than themolecular weight of high molecular weight polyethylenes, and which isabout 2 million to about 10 million grams/mole, preferably about 3million to about 6 million grams/mole. Unlike high molecular weightpolyethylenes, which generally have a molecular weight of about 400,000to about 600,000 grams/mole, the UHMW polyethylene is typically not meltprocessable. Balloons formed from this material exhibit compliantexpansion at relatively low strains and exhibit substantially lesscompliance at higher strains.

The node and fibril structure of the UHMW polyethylene causes it toexhibit essentially compressible deformation at relatively smallstrains, with a low Young's modulus. At high strains, the UHMWpolyethylene balloons of the invention preferably exhibit low compliancedue to rearrangement in the microstructure. Embodiments of the inventionsuited to intravascular applications preferably exhibit compliant radialexpansion of about 100% to about 400% of the uninflated diameter, atpressures up to about 6 to about 8 atm. Once expanded, the balloonsexhibit relatively low compliance at pressures above 8 atm and can havea burst pressure of at least about 18 atm. For stent deliveryapplications, the polyethylene preferably has a foamlike compressiblestate at low strains so that the stent can be crimped onto the balloonwith good retention.

Balloon catheters of the invention generally comprise an elongated shaftwith at least one lumen and a UHMW polyethylene balloon on a distalshaft section with an interior in fluid communication with the shaftlumen. The balloon catheters of the invention may be configured for avariety of uses, such as angioplasty or stent delivery. A stent deliverycatheter employs a balloon having the characteristics of the inventionto deploy the stent. Preferably, the oriented polyethylene exhibits afoam-like compressible state at low strains, facilitating crimping ofthe stent onto the balloon with improved stent retention. In accordancewith the invention, the stent may be provided with a stent covergenerally comprising a tubular sheath formed of the UHMW polyethyleneand configured to be disposed on an outer and/or inner surface of thestent and implanted with the stent in the patient's vessel.

Vascular grafts of the invention generally comprise a tubular bodyformed of the UHMW polyethylene. The vascular graft is configured to beimplanted in a patient, and may be used for a variety of proceduresincluding anastomosis, bypass surgery, and aneurysm repair.

The invention also comprises methods of forming a medical devicecomponent such as a balloon, stent cover or vascular graft, frommicroporous polyethylene having an oriented node and fibril structure.Generally, the method comprises the steps of compacting ultrahighmolecular weight polyethylene powder, deforming the compactedpolyethylene to render the polyethylene microporous and to impart anoriented node and fibril structure to the polyethylene, and forming themedical device component from the polyethylene. Optionally, the powdercan be sintered prior to deformation. Also optionally, the orientedpolyethylene can be heat set. Preferably, a tubular medical devicecomponent such as a balloon may be formed by wrapping a sheet of theoriented polyethylene around a mandrel to form a tube and then heatfusing the polyethylene layers together, or by directly producing anoriented tubular member.

The medical devices such as catheter balloons, stent covers, andvascular grafts of the invention have improved performance due to theUHMW polyethylene which is microporous, biocompatible, and biostable,and which has excellent mechanical properties. Further, UHMWpolyethylene is compatible with electron-beam (i.e., e-beam)sterilization, unlike expanded polytetrafluoroethylene (i.e., ePTFE),which degrades when exposed to e-beams. As a result, medical devicessuch as balloons of this invention can be expanded compliantly to theirworking diameter but exhibit substantially less compliance at greaterpressures, providing control over expansion even at pressures suitablefor conventional intravascular procedures such as angioplasty or stentdelivery. Further, the formed-in-place balloons of the invention havesufficient strength to improve the safety of conventional intravascularprocedures. UHMW polyethylene also facilitates device manufacture,because the processing temperatures for polyethylene are relatively low,and the polyethylene can be heat bonded or attached with adhesives toother device components.

These and other advantages of the invention will become more apparentfrom the following detailed description when taken in conjunction withthe accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of a ballooncatheter for delivering a stent that embodies features of the invention.

FIG. 2 is a transverse cross-section of the catheter shown in FIG. 1taken at line 2—2.

FIG. 3 is a transverse cross-section of the catheter shown in FIG. 1taken at line 3—3, showing the stent disposed over the inflatableballoon.

FIG. 4 is an elevational view, partially in section, of a vascular graftor stent cover which embodies features of the invention.

FIG. 5 is a transverse cross-section of the graft or cover shown in FIG.4, taken along lines 5—5.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-3 illustrate an over-the-wire type stent delivery ballooncatheter 10 embodying features of the invention. Catheter 10 generallycomprises an elongated catheter shaft 12 having an outer tubular member14 and an inner tubular member 16. Inner tubular member 14 defines aguidewire lumen 18 adapted to slidingly receive a guidewire 20. Thecoaxial relationship between outer tubular member 14 and inner tubularmember 16 defines annular inflation lumen 22. An inflatable balloon 24disposed on a distal section of catheter shaft 12 having a proximal endsealingly secured to the distal end of outer tubular member 14 and adistal end sealingly secured to the distal end of inner tubular member16 so that its interior is in fluid communication with inflation lumen22. An adapter 26 at the proximal end of catheter shaft 12 is configuredto direct inflation fluid through arm 28 into inflation lumen 22.

In the embodiment illustrated in FIG. 1, an expandable stent 30 ismounted on balloon 24. The distal end of catheter may be advanced to adesired region of a patient's lumen 32 in a conventional manner andballoon 24 may be inflated to expand stent 30, seating it in the lumen.

In the embodiment illustrated in FIG. 1, the balloon 24 has a layer 34formed from an elastomeric material. In the preferred embodimentillustrated, elastomeric layer 34 is on the interior of balloon 24,although in other embodiments it may be on the exterior or the balloon24. Elastomeric layer 34 expands elastically to facilitate deflation ofthe balloon 24 to its preinflation diameter and shape, and can alsolimit or prevent leakage of inflation fluid through the microporouspolyethylene.

Balloon 24 is formed at least in part of a UHMW polyethylene.Preferably, the UHMW polyethylene has a molecular weight of about 3million to about 6 million. Suitable UHMW polyethylenes are availablefrom Hoechst Celanese, and described in Ultrahigh Molecular WeightPolyethylenes (UHMWPE), Engineered Materials Handbook, Vol. 2:Engineering Plastics, H. L. Stein, and WO 91/01210, incorporated byreference herein in its entirety. Presently preferred UHMW polyethylenesare classified by molecular weight determinations detailed in ASTM(American Society for Testing and Methods) D 1601 and D 4020. In apresently preferred embodiment, the UHMW polyethylene is processed sothat it is microporous and exhibits an oriented structure comprisingnodes interconnected by fibrils. The microporous UHMW polyethylene withan oriented node and fibril microstructure has a porosity of about 20%to about 90%, and an internodal distance, also expressed as fibrillength, of about 5 μm to about 200 μm. Examples of microporous UHMWpolyethylenes, having a node and fibril microstructure and a suitablyhigh orientation, are described in WO 91/01210, incorporated byreference herein in its entirety. As described in WO 91/01210, such UHMWpolyethylene materials may exhibit a negative Poisson ratio. Balloonsformed from this material exhibit compliant expansion at relatively lowstrains and exhibit substantially less compliance at higher strains. Forexample, in a presently preferred embodiment, balloon 24 expandscompliantly by about 100% to about 400% of the uninflated diameter atpressures of about 6 to about 8 atm. Once expanded, the balloon 24 isrelatively noncompliant at pressures greater than about 8 atm, up to theburst pressure of the balloon which preferably is at least about 18 atm.

In the embodiment illustrated in FIG. 1, a stent cover 40 formed of theUHMW polyethylene is disposed on an outer surface of the stent 30. Asdiscussed above, the UHMW polyethylene forming the stent cover 40 can beprocessed to be microporous with a node and fibril microstructure. Stentcover 40 is secured to the surface of the stent 30 before the stent isintroduced into the patient's vasculature, and expanded, together withthe stent, to implant the stent and stent cover thereon in the vessellumen. Stent cover 40 secured to the stent has a generally tubularstructure conforming to a surface of the stent. In the presentlypreferred embodiment illustrated in FIG. 1, the stent cover 40 extendsthe length of the stent 30. However, in alternative embodiments thestent cover may have a length longer than or shorter than a length ofthe stent. The stent cover 40 length may be selected to fit a variety ofconventionally sized stents, with a typical diameter of about 2 mm toabout 10 mm. The stent cover 40 wall thickness is typically about 20 μmto about 400 μm, preferably about 40 μm to about 100 μm. The stent cover40 provides a biocompatible, biostable surface on the stent, whichreduces plaque prolapse through the stent struts. A stent cover may beprovided on an inner surface of the stent (not shown).

In another embodiment of the invention illustrated in FIG. 5, vasculargraft 50 comprises a tubular body 51 having a lumen 52 therein, formedof an UHMW polyethylene. Ports 53,54 are at either end of the graft 50.As discussed above the UHMW polyethylene can be processed to bemicroporous with a node and fibril microstructure. The graft isconfigured for being implanted in the patient, and it may be expandedinto place within a vessel or surgically attached to a free end of avessel. The graft 50 length is generally about 4 to about 80 mm, andmore specifically about 10 to about 50 mm, depending on the application,and wall thickness is typically about 40 μm to about 2000 μm, preferablyabout 100 μm to about 1000 μm. The diameter is generally about 1 toabout 35 mm, preferably about 3 to about 12 mm, depending on theapplication.

A process of forming the microporous node and fibril structure of theUHMW polyethylene generally comprises compacting polyethylene powder andthen deforming it to impart the node and fibril structure. The step ofcompacting the polyethylene powder can by any suitable means includingthe presently preferred embodiments of applying pressure, with orwithout additional heat, or forming a slurry with a lubricating mediumand then extruding the slurry through a die. The lubricating mediumshould be evaporated from the slurry after extrusion. When applyingpressure and heating the polymer powder, the polyethylene may be heatedto a temperature at or above its softening point, but below its meltingpoint, to sinter the material. For example, preferred UHMW polyethyleneblends have a sintering temperature of about 160° C. The size and shapeof the polyethylene particles can be chosen to influence the node andfibril structure and optimize the properties of the resulting material.For example, the shape of the particles tends to be reflected in theshape of the nodes.

The particle size is generally about 0.1 μm to about 250 μm, preferably0.1 μm to about 40 μm.

The compacted powder is then deformed to impart the oriented node andfibril structure. Typically, the polyethylene is deformed by stretchingor by extrusion through a die. The deformation step may be performedeither at ambient or elevated temperatures. In a preferred embodiment,sintered polyethylene is ram extruded at ambient temperature to form asheet of material, which is then rapidly stretched, axially orbi-axially, to orient the structure. Optionally, the stretched materialcan be heat set. The processing of the polyethylene also renders itmicroporous, and the amount of stretch experienced by the materialcontrols the distance between the nodes and the corresponding fibrillength.

One presently preferred method of forming the UHMW polyethylenegenerally comprises preparing a homogeneous paste of UHMW polyethylenein a low boiling mineral oil. The paste is then compacted into a billetby applying pressure and optionally applying heat. The billet is thenloaded into a ram extruder and a tube or film is extruded. The extrusionmay be done at room temperature, or the temperature may be elevated. Theoil is then evaporated from the UHMW polyethylene by heating the film toa temperature not exceeding the melting point of the UHMW polyethylene.The film or tube is then uniaxially or bi-axially oriented to producethe oriented node and fibril structure. The oriented tube may thenoptionally be heat set at temperatures just above the melting point ofUHMW polyethylene, which has a crystalline melting point of about130-140° C.

Another presently preferred process comprises compacting UHMW ethyleneparticles into a billet at temperatures below the melting point.Preferably, this step would be done at about 100-120° C. Preferably, thepressure applied is about 0.01 GPa to about 0.08 GPa. The billet issintered at temperatures above the crystalline melting point withoutapplying any pressure. This step is completed at a preferred temperatureof about 130-160° C. The sintered billet is extruded through a film orannular die in a ram extruder. The UHMW polyethylene is then optionallyoriented and heat set as described above.

Generally, the balloons of the invention are formed from the sheet ofstretched material. The material is wrapped around a mandrel to form atube and then heated to fuse the wrapped material together. Theresulting parison may be secured to the conventional catheter componentsby laser bonding or plasma treatment followed by adhesive bonding.

The dimensions of catheter 10 are determined largely by the size of theguidewires to be employed and the size of the artery or other body lumenthrough which the catheter must pass or the size of the stent beingdelivered. Typically, the outer tubular member 14 has an outer diameterof about 0.02 to about 0.04 inch (0.05 to 0.10 cm), usually about 0.037inch (0.094 cm), an inner diameter of about 0.015 to about 0.035 inch(0.038 to 0.089 cm), usually about 0.03 inch (0.076 cm). The wallthickness of the outer tubular member 14 can vary from about 0.002 toabout 0.008 inch (0.0051 to 0.0201 cm), typically about 0.003 inch(0.0076 cm). The inner tubular member 15 typically has an outer diameterof about 0.012 to about 0.016 inch (0.030 to 0.041 cm), usually about0.014 inch (0.036 cm). The overall length of the catheter 10 may rangefrom about 100 to about 150 cm, and is typically about 135 cm.Preferably, balloon 24 may have a length about 0.5 cm to about 4 cm andtypically about 2 cm with an inflated working diameter of about 1 toabout 8 mm. Inner tubular member 16 and outer tubular member 14 can beformed by conventional techniques, for example by extruding, frommaterials already found useful in intravascular catheters such apolyethylene, polyvinyl chloride, polyesters, polyamides, polyimides andcomposite materials. The various components may be joined by heatbonding or use of adhesives.

While the present invention is described herein in terms of certainpreferred embodiments, those skilled in the art will recognize thatvarious modifications and improvements may be made to the inventionwithout departing from the scope thereof. For example, in the embodimentillustrated in FIG. 1, the catheter is a stent delivery catheter.However, one of skill in the art will readily recognize that theballoons of this invention may also be used with other types ofintravascular catheters, such as over-the-wire and rapid exchangedilatation catheters. Moreover, although individual features of oneembodiment of the invention may be discussed herein or shown in thedrawings of the one embodiment and not in other embodiments, it shouldbe apparent that individual features of one embodiment may be combinedwith one or more features of another embodiment or features from aplurality of embodiments.

What is claimed is:
 1. A balloon for an intraluminal catheter, whereinthe balloon comprises ultrahigh molecular weight polyethylene.
 2. Theballoon of claim 1, wherein the polyethylene is a microporouspolyethylene having a node and fibril microstructure comprising nodesinterconnected by fibrils.
 3. The balloon of claim 2, wherein thepolyethylene has an internodal distance of about 5 μm to about 200 μm.4. The balloon of claim 1, wherein the polyethylene has a molecularweight of about 3 million to about 6 million.
 5. The balloon of claim 1,wherein the polyethylene is formed of particles having a size of about0.1 μm to about 250 μm.
 6. The balloon of claim 1, wherein thepolyethylene is formed of particles having a size of about 0.1 μm toabout 40 μm.
 7. The balloon of claim 1, wherein the polyethylene has aporosity of about 20% to about 90%.
 8. The balloon of claim 1, whereinthe polyethylene exhibits volumetric compressibility in the uninflatedstate.
 9. The balloon of claim 1, wherein the balloon expandscompliantly at pressures below about 8 atm and substantially lesscompliantly at pressures above about 8 atm.
 10. The balloon of claim 9,wherein the balloon has an outer diameter which expands by about 100% toabout 400% of the uninflated diameter at pressures below about 8 atm.11. The balloon of claim 1, wherein the balloon further comprises anelastomeric layer.
 12. The balloon of claim 11, wherein the elastomericlayer is an inner layer of the balloon.
 13. The balloon of claim 1wherein the ultrahigh molecular weight polyethylene has a negativePoisson ratio.
 14. The balloon of claim 1 wherein the ultrahighmolecular weight polyethylene exhibits microstructural rearrangementduring inflation of the balloon.
 15. The balloon of claim 1 wherein themicrostructural rearrangement occurs at balloon inflation pressuresabove 8 atm.
 16. A balloon for an intraluminal catheter, wherein theballoon comprises ultrahigh molecular weight polyolefin.